Unify the way optional params are displayed.

LDoc has two different ways to annotate optional parameters, [opt] and [type=?foo]. This patch attempts to resolve the differences in the way optional parameters display: [opt] params are shown as "optional" in the param description, and [type=?] params are shown in brackets in the param list. Also added a --plain_params setting to turn off brackets and default values in the param list. 

- Params with modifiers like [type=?foo] have brackets in param list.
- Params with modifiers like [opt] show up as "optional" in param description.
- Add -P,--plain_params setting for no brackets or defaults in param list.